The goal of this project is to study the in vivo and in vitro biological effects on hematopoiesis of recombinant human stem cell factor (rhSCF), a ligand for the human c-kit molecule, a cell surface receptor with tyrosine kinase activity. In vitro, SCF by itself stimulates little proliferation of human and baboon marrow hematopoietic colony-forming cells (CFC), but does have stimulating effects when combined with other hematopoietic growth factors, including IL-3, G-CSF, GM-CSF, and Epo. In vivo, SCF stimulates a dose-dependent leukocytosis, with increases in neutrophils, monocytes, eosinophils, basophils, and T and B lymphocytes in the blood. SCF also stimulates a reticulocytosis and increases in the numbers of red blood cells and in hematocrit. In marrow, SCF stimulates a dose-dependent increase in cellularity and morphologically immature erythroid and granulocytic cells. Importantly, SCF increases the number of hematopoietic colony-forming progenitor cells in marrow and blood. The number of progenitor cells in blood increases from 10-fold to >1000-fold in animals given SCF at a dose of 200 g/Kg/day, with an associated increase in numbers of CD34+ cells in blood. We have also shown that transplantable cells capable of rescuing and reconstituting lymphohematopoiesis in lethally irradiated baboons are increased in the blood by treatment with high doses of SCF. SCF also has in vivo synergy with G-CSF. SCF at 25 g/kg/day, which alone does not stimulate a leukocytosis or an increase in peripheral blood progenitors, when administered with G-CSF increases the rate at which circulating WBC increase as well as the maximum WBC achieved, compared with G-CSF alone. Most importantly, this dose of SCF when combined with G-CSF dramatically enhances the circulation of progenitor cells as well as marrow repopulating cells compared with G-CSF alone. Animals transplanted with blood cells collected during stimulation with the combination of SCF plus G-CSF engraft much more rapidly than do animals transplanted with cells collected during stimulation with G-CSF alone. Studies are in progress to determine if SCF, in combination with other growth factors, can be used to expand the number of progenitors from marrow and blood in vitro and if these expanded cells can engraft lethally irradiated baboons.